Provide both signed and unsigned int variants of bitcode-based builtins.

When creating function Symbols for functions that were defined in LLVM bitcode for the standard library, if any of the function parameters are integer types, create two ispc-side Symbols: one where the integer types are all signed and the other where they are all unsigned.  This allows us to provide, for example, both store_to_int16(reference int a[], uniform int offset, int val) as well as store_to_int16(reference unsigned int a[], uniform int offset, unsigned int val). functions.

Added some additional tests to exercise the new variants of these.

Also fixed some cases where the __{load,store}_int{8,16} builtins would read from/write to memory even if the mask was all off (which could cause crashes in some cases.)

tests/store-int16-1.ispc

@@ -0,0 +1,16 @@
+export uniform int width() { return programCount; }
+
+export void f_fu(uniform float RET[], uniform float aFOO[], uniform float b) {
+    uniform int x[16];
+    for (uniform int i = 0; i < 16; ++i)
+        x[i] = 0xffffffff;
+    unsigned int val = aFOO[programIndex];
+    store_to_int16(x, 5, val);
+    unsigned int v = load_from_int16(x, 6);
+    RET[programIndex] = v;
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 2+programIndex;
+    RET[programCount-1] = (unsigned int)0xffffffff;
+}